Some computers use a touch panel as an input device. The touch panel is an input device provided with an input surface, and outputs a digital signal having a predetermined bit length indicating, in a predetermined resolution, a coordinate of a position touched by a user (user-specified position) on the input surface to a touch panel control device of the computer. Patent Document 1 and Patent Document 2 disclose specific examples of the touch panel.
Recently, the resolution of touch panels has increased, and, accordingly, the bit length of the digital signal output from the touch panel has increased. Specifically, there are touch panels which indicate one coordinate in ten bits.    Patent Document 1: JP 2000-20474 A    Patent Document 2: JP 10-312257 A
Moreover, some recent computers function as a display function-equipped coordinate input device in which an input surface (such as touch panel) of the coordinate input device is used as display means such as a display device. The display function-equipped coordinate input device consumes a large amount of power especially when the display function carries out displaying, and, when the computer is used as the display function-equipped coordinate input device, a reduction in power consumption becomes a problem, and, in order to solve this problem, various attempts have been made.
In an example, there are provided a normal operation mode, and a low power consumption mode for restricting the power consumption by stopping the display function. The normal operation mode transitions, when a user-specified operation has not been detected on an input surface for a predetermined period of time, to the low power consumption mode, and, when the user-specified operation is detected on the input surface, the low power consumption mode transitions to the normal operation mode (refer to, for example, Patent Document 3).    Patent Document 3: JP 10-111749 A
Moreover, an analog resistive touch panel has two resistive films for respectively detecting an X-axis position and a Y-axis position, and detects a touch position by means of resistance division of the resistive films for the X axis and the Y axis. Referring to drawings, a description thereof is now given.
FIG. 19 is a diagram showing an equivalent circuit for an X-axis position detection circuit or a Y-axis position detection circuit of an ideal analog resistive touch panel 3400. As shown in the drawing, the touch panel 3400 is provided with a resistive film (resistance RT) from one end to the other end, and a DC voltage VD is applied on the resistive film RT. The touch panel 3400 is configured so that, when a touch surface is touched, a voltage V applied at a depressed position is acquired. According to the voltage V thus acquired, a resistance RT1 from a grounded point to the touch position (resistance dividing point) is calculated. Specifically, the resistance RT1 is calculated by the following Equation 1.RT1=RT×V/VD  (1)
The touch position of the touch panel 3400 is calculated based on the resistance RT and the resistance RT1 thus acquired respectively for the X axis and the Y axis.
The touch panel 3400 shown in FIG. 19 is the ideal touch panel which does not have electric resistances in wiring. However, the wiring actually has electric resistances, and an equivalent circuit thereof is represented as a touch panel 3401 (FIG. 20). In this case, the resistance RT1 is given by Equation 2. It should be noted that V1 and V2 are respectively a voltage applied on an end on the ground point side of the resistor RT, and a voltage applied on an end on a power supply side of the resistor RT.RT1=RT×(V−V1)/(V2−V1)  (2)
In this way, actually, the resistance RT1 is calculated based on the voltages (V1 and V2) applied on both ends of the resistive film.
It should be noted that resistances R1 and R2 of the wiring shown in FIG. 20 change according to the use environment and the period of use. When those changes occur, the voltages applied on both ends of the resistive film change accordingly. Thus, the voltages (V1 and V2) applied on both ends of the resistive film are measured and saved when the power supply is turned on, upon a reset, when the stop mode is released, or when a touch operation has not been carried out, for example, the touch input or command transmission has not been carried out for thirty seconds, and are used for the calculation according to Equation 2 when a touch operation is carried out.
Further, in a technology described in Patent Document 4, presence or absence of a touch operation is detected periodically (at intervals of 50 ms), voltages applied on both ends of resistive films are measured and saved when a touch operation is not carried out, and, when a touch operation is carried out, the saved voltages are used as correction data for the touch position.    Patent Document 4: JP 09-160719 A